Curable thermosetting resin compositions containing, polyphenylene ether (hereinafter referred to as PPE) are known in the art as useful dielectrics. Such compositions, generally in the form of fiber reinforced prepregs (i.e., substrates impregnated with uncured or partially cured resins), undergo cure to form materials with low dielectric constants and other favorable properties, including solvent resistance and solder resistance. Such materials are ideal for use as, for example, copper-clad laminates suitable for etching to form printed circuit boards. The initial production of the fiber reinforced prepreg is an important part of the lamination process and generally involves infusing a solution of the resin into a fibrous substrate such as glass.
When employing a resin solution, a number of solvents have been used including, for example, dichloromethane, chloroform, trichloroethylene, benzene, toluene and xylene. In practice, these solvents, can be used individually or in combination to dissolve the polyphenylene ether composition. Because of deleterious health effects associated with halogenated hydrocarbons and benzene, it is preferable to use substituted aromatic hydrocarbons, in particular toluene, as the solvent for prepreg manufacture. When employing a polyphenylene ether-thermosetting resin composition such as a polyphenylene ether-epoxide composition using an inert organic solvent such as toluene to prepare printed circuit boards, gelation of the resin solution generally occurs at room temperature. The gelation is believed to be PPE that is separating from the organic solution yet remains highly swelled by the solvent. A number of PPE compositions are known to exhibit this phenomenon including polyphenylene ether-epoxide compositions described in U.S. Pat. No. 5,162,450 and Japanese patent application 6[1994]-200054.
The polyphenylene ether-epoxide containing prepregs described in U.S. Pat. No. 5,162,450 need to be maintained at about 55.degree. C. to inhibit PPE gel formation and maintain a well behaved fluid. The aforementioned Japanese patent application 6[1994]-200054, teaches that PPE gelation (translated as waxification) can be overcome by dissolving the PPE at elevated temperatures (80-88.degree. C.) and infusing the glass cloth at (45-50.degree. C.).
We have unexpectedly found that prepregs can be produced from toluene solutions of polyphenylene ether-thermosetting resins at room temperature (ca. 23.degree. C.) without any observed gelation if the number average molecular weight of the PPE in the mixture is maintained at below 3,000.
Commercially available PPE vary in number average molecular weight from roughly 15,000-25,000. Lower molecular weight PPE have been used, such as those employed in U.S. Pat. No. 5,162,450 which discloses a number average molecular weight of about 3,000 to about 15,000 and, preferably, 5,000-10,000. However, even though the PPE component of the U.S. Pat. No. 5,162,450 is a relatively low molecular weight (3,000-15,000), PPE gelation nevertheless occurs upon standing at room temperature in solution. Moreover, in the cured state two distinct phases, corresponding to PPE rich and polyepoxide rich domains, are observed.
Japanese Patent Disclosure Publication No. 1983-219217 describes a relatively low number average molecular weight PPE in combination with an epoxy resin potentially useful as dielectrics for printed circuit boards. The number average molecular weight of the PPE disclosed therein is less than 10,000 and, preferably, 1,000-8,000. However, it does not describe the use of such a composition in a substituted hydrocarbon solvent such as toluene, and mentions the use of high temperatures with respect to other aromatic solvents. The example in this reference is directed to a molding composition and the molecular weight of the PPE disclosed in the Example has a number average molecular weight greater than about 5,000. The polymer described in the Example, in a substituted hydrocarbon solvent such as toluene, will gel upon standing at room temperature and would, therefore, require maintaining the resin solution at elevated temperatures. In the cured state, it is expected that the resin system would not be homogenous as in the instant invention which employs a PPE having a specific number average molecular weight of less than 3,000.
Thus, the composition of the prior art requires maintaining the PPE containing solution at elevated temperatures that can afford environmental and health risks associated with the solvent vapors. Lowering the solvent temperature in the prior art would lead to gelation and separation of the PPE in the organic solution. The cured PPE containing thermosets of the prior art additionally exhibit undesirable phase separation that results in less than optimal physical properties. It is therefore apparent that a nened continues to exist for improved PPE-thermoset compositions.